Titanium Silicalites are crystalline porous materials. They are made up of tetrahedrallycoordinated Silicon and Titanium atoms surrounded by four Oxygen atoms. Said tetrahedrons of Silicon and Titanium are link up to form rings through corner sharing of Oxygen atoms. Such rings further link up to form three-dimensional Titanium Silicalite structures. Said rings define the pores that give these materials their characteristic properties as molecular sieves, adsorbents and catalysts. The size, shape and other parameters of said rings and of the associated pores determine the selectivity, activity and other properties of the catalyst. As far as this specification is concerned, the term ‘shaped’ is intended to cover transformed TS-1 (From raw TS-1 powder) in the form of an extrudate, tablet/pellet product that may be further sized to a granulated product. Adoption of other methods of fabrication is within the scope of the invention.
In this specification, unless otherwise required by the context, the term ‘Titanium Silicalite-1’ or ‘TS-1’ or ‘Ti-ZSM-5’ or ‘Ti-MFI’ refers to the raw form which is obtained through any of the hydrothermal and other processes for making raw Titanium Silicalite-1 reported in the art. Said processes for raw Titanium Silicalite-1(TS-1) yield the TS-1 in a lump or powder form. The material-in-process at the different steps of a process for making shaped TS-1 product is also referred to as TS-1 material. The meaning appropriate to the context may be adopted. The product obtained after said shaping/forming operation is referred to herein as the ‘shaped Titanium Silicalite-1 product’ or in short as ‘shaped TS-1 product’ or ‘TS-1 product’ or “TS-1 catalyst” or “TS-1 extrudates” or “TS-1 tablets” or “TS-1 pellets”. It is also referred to herein at some places as ‘Shaped TS-1’ or “formed TS-1’ or “TS-1 catalyst” or ‘formed Titanium Silicalite-1 product’.
Titanium Silicalite (TS-1) comprises Ti(IV) and is an oxidation catalyst that has wide application in the organic transformations. It is a heterogeneous catalyst that is used in numerous oxidation reactions. It has good activity, selectivity and stability. It is usually used in conjunction with other oxidizing agents, such as for example, hydrogen peroxide with which it works particularly well. Some of the major reaction types wherein TS-1 catalyst is used are:                1) Epoxidation of olefins; oxidation of Propylene to Propylene oxide/Glycols and corresponding ethers; Allyl chloride to Epichlorohydrin,        2) Hydroxylation of aromatics; hydroxylation of phenol to Catachol and Hydroquinone;        3) Oxidation of alcohols; Alcohols to corresponding aldehydes or ketones        4) Oxidation of hydrocarbons; Parafins to corresponding alcohols/ketones.        5) Ammoxidation of Cylohexanone to Cyclohexanoneoxime, etc.,        
Since the reactants and reaction-conditions of diverse industrial processes employing TS-1 catalysts are different, each of these processes markedly warrants unique/specific physico-chemical attributes and dimensions of the shaped TS-1 catalyst, or particular combinations thereof. However, the available processes of manufacture and fabrication of TS-1 products in the art adopt a one-size-fits-all-approach whereby the physico-chemical attributes and dimensions of the TS-1 product so manufactured are stereotypically restricted to a rather narrow range of combination-attributes. This is especially due to the lack of versatile recipes and/or process-engineering techniques in the art to tailor-make or optimize the attributes of the end product to suit the requirements of a given industrial process. The obvious result of this lacuna is a direct compromise on quality as well as resource economics of the industrial processes employing TS-1 catalysts. This is because the effective reaction rate depends not only on the temperature and the concentration of the reactants, but also on macrokinetic parameters such as phase boundary, bulk density, particle size of the catalyst, pore structure and the transport rate in the diffusion boundary layer. If the physical reaction steps are rate determining, then the catalyst capacity is not fully exploited due to an unsuited or incompatible physical configuration of the catalyst. The nature of crushing strength, bulk density and pore volume distribution and Ti availability, which are more concerned with the subject matter of this invention are explained as follows:
Crushing Strength:
Solid catalysts are usually dispatched in drums. During transport, storage and loading there is risk of damage to the catalyst from mechanical means. Even when the catalyst is placed in a fixed-bed reactor, the catalysts in each layer have to carry the weight of the catalysts in the above-lying layers. The catalyst should be able to resist the mechanical or other such stress. Hence crushing strength is measured to meet the requirement from end-user for a particular process. Crushing Strength (sometimes also referred to as “side crush strength”) in general is measured by first placing a single piece of catalyst horizontally between two parallel plates or blocks and thereafter loading the assembly so that the piece is compressed and finally broken. A measure of the load required for cracking or breaking of a particle/unit is the crushing strength.
Bulk Density:
The bulk density of a catalyst is determined by measuring the volume of a known mass of catalyst sample. Whenever a process is scaled up to pilot or commercial scale, the form in which a catalyst is used is decisive for the ultimate performance of the unit. The catalyst thus will perform in a range of different physico-chemical properties (typically referred as catalyst specification). As far as bulk density is concerned, it is affected by the density of powder particles (used for forming/shaping) and the spatial arrangement of particles in the formed catalyst. Hence for guaranteed performance, a catalyst is supplied to the end-user with a range of bulk densities suitable in a particular process.
Pore Volume Distribution:
In general pore volume distribution can influence the reactions which are diffusion controlled. Pore volume distribution of the catalyst is one of the factors that influence the rate of the reaction.
The porous nature of the catalyst contributes substantially to the active surface area at which the catalytic reaction takes place. For the internal surfaces of the catalyst body (extrudates tablets/pellets) to be utilized effectively, the feed (reactants) must diffuse through the pores to reach the internal surfaces, and the reaction products must diffuse away from those surfaces and out of the catalyst body. The resistance to internal diffusion in the catalyst bodies can become a rate limiting factor in the reaction. Hence to overcome resistance to internal surfaces, the pore structure/pore volume of the catalyst is modified accordingly.
Ti Availability:
Ti(IV) in the TS-1 powder is responsible for catalytic activity in a given application. In the forming/shaping process, the more the content of TS-1 powder in forming/shaping recipe, the more active sites are accessible/available for the reactants.
It follows therefore that an optimal ratio of TS-1 Binder component has to be reached in order to maximize Ti availability while simultaneously sustaining superior degrees of crushing strength, bulk density and pore volume. For example, the catalyst with 10% binder component will have more active sites than a catalyst with 20% binder component.
However, the optimal ratio of TS-1 and Binder may not be same for different applications. In general, if the application demands more Ti availability then the catalyst should contain either no binder or minimum binder to give the desired physico-chemical properties and performance. Owing to the requirement of specific applications, the Ti content in the catalyst needs to be fine-tuned to suit the applications. Besides, more Ti in catalyst may not always be necessary and/or desirable for all industrial applications. For example, more Ti in some oxidation applications leads to more exothermicity, which may result in decomposition of the oxidant. In such cases it is desirable to reduce the Ti content (i.e. Ti availability) to suit a particular application.
There has always been a gap between the configuration-demands of the industry and the state of the art-supply of TS-1 (specifications/attributes) due to the said one-size-fits-all-approach in the state of the art. For example the requirement for bulk density and crushing strength for epoxidation of propylene to propylene oxide may not be the same as required for epoxidation of Allyl chloride to Epichlorohydrin. There is therefore an urgent and long-felt need for process-engineering techniques that can custom-make or at least simultaneously vary or optimize a plurality of attributes of the shaped TS-1 product to suit the requirements of diverse industrial processes that employ Titanium based catalysts.
Yet another problem faced especially by manufacturers of shaped TS-1 product is the inflexibility and inadaptability of the prevalent process recipes in the art, owing to especially (a) the tedium of the long-drawn-out, energy intensive mechanical, physical and chemical process steps that have evolved thus far in the art, and also (b) the narrow range of alternatives to choose from as regards the ingredients that go into transformation of raw TS-1 to the shaped TS-1 extrudates, tablets/pellets or granules.
In the process for forming the TS-1 extrudate (also referred to as formed TS-1) disclosed in U.S. Pat. No. 6,551,546, Tetra Ethyl Ortho Silicate is hydrolyzed, and the ethanol generated by hydrolysis is removed by distillation to obtain silica sol. The sol is used as binder along with TS-1 for making microspheres (by Spray drying). The raw TS-1 material in the form of a spray dried powder is mixed with the proprietary binder Ludox AS-40 (DuPont, now Grace) which is a colloidal silica binder. Methylcellulose and water are also added to convert the TS-1 composite into a paste. Optionally fumed silica/PEG (polyethylene glycol) are also added, and TS-1 is densified prior to shaping.
In U.S. Pat. No. 6,849,570, the binders used either contain Al or Na or other cationic impurities, which are not desirable for oxidation reactions performed using hydrogen peroxide as oxidant, since the presence of Al or Na or other trace metal impurities is detrimental as it accelerates the decomposition of hydrogen peroxide, thereby resulting in either poor efficiency and/or inferior performance.
In the process disclosed in US application 20030130116, Methyl cellulose and Alumina are added to spray dried raw TS-1 powder. Requisite amount of water is incorporated to obtain a paste which is kneaded. Partially hydrolyzed Tetra Ethyl Ortho Silicate (Ester 40 from Wacker, 40% SiO2) is used as binder. The ethanol generated by hydrolysis is removed from the reaction mixture since 40 wt % silica binder can only be produced from Tetra Ethyl Ortho Silicate when the ethanol is evaporated.
In the process disclosed in EP1071506B1 and U.S. Pat. No. 6,551,546 B1, Tetra Ethyl Ortho silicate is hydrolyzed, and ethanol generated by hydrolysis is removed by distillation to obtain silica sol. This sol is used as binder with TS-1 to form a spray dried material. The spray dried material is further shaped using binders such as Silica Sol, Ludox AS-40.
In U.S. Pat. No. 6,106,803, the process for preparing Titanium Silicalite-1 granulates comprises, hydrolyzing TEOS with alkyl ammonium hydroxide. Subsequently mixing with TS-1 powder and spray drying to obtain Titanium Silicalite-1 microgranules.
Prior art thus reveals long-drawn-out processes which are not only time consuming but also resource intensive and cost prohibitive due to the following disadvantages, among others;                (a) US 20030130116 A1, EP1071506B1, U.S. Pat. No. 6,551,546 B1, among others, use Tetra Ethyl Ortho silicate (TEOS) as binder precursor which is expensive.        (b) Further, when the Tetra Ethyl Ortho silicate is hydrolyzed, ethanol generated by hydrolysis needs to be removed. This is a time consuming and resource intensive step that substantially increases process cost.        (c) Other commonly used binders and forming aids used in prior art include, silica precursors such as Ludox AS-40, Silica sol, Tetra-n-propylammonium hydroxide, etc., wherein the Na content and other metal impurities are high.        (d) Spray drying (prior to forming, extrusion etc.,) is an integral step in prior art processes which is not only time consuming but also resource intensive and thereby increases process costs.        (e) A further problem faced by manufacturers of shaped TS-1 product is the constraints imposed by certain process steps employed in the making of shaped TS-1 using raw TS-1 powder which inflict a restrictive influence on the versatility of the TS-1 product so manufactured. For example, as also documented in prior art literature [including US 20030130116 A1], extrusion to turn out shaped TS-1 product is not possible with calcined Titanium silicate, which has proved to be a major stumbling block where TS-1 extrudtaes are required to be manufactured. This is again due to the shortcomings in the process-recipes in vogue which impose this, among other, restrictions. In case of the process used especially by US 20030130116, the calcined TS-1 already contains silica sol which is added prior to spray drying the TS-1. Hence the desired rheology is not achieved in the paste, which may be responsible for not achieving extrution with calcined TS-1.        
There is therefore an urgent and long-felt need for a versatile recipe and a process that ensures economics of both time and resources, and also provides for custom-making or at least varying/optimizing the combination-attributes of shaped TS-1 product to match the requirements of diverse industrial processes that employ Titanium based catalysts.
These inventors have, after extensive research, devised (a) an abbreviated (shortened) process recipe (b) comprising a novel binder composition (c) which lends surprising versatility to the process wherein, the said process can be manipulated to custom-make shaped TS-1 extrudates, tablets or pellets, whereby their physico-chemical attributes such as but not limited to crushing strength, bulk density, Ti availability and prore volume distribution etc., can be individually engineered to suit specific requirements of diverse industrial processes employing Titanium based catalysts.
To overcome the disadvantages of prior art processes, alternate recipes comprising exotic binder compositions were explored by these inventors who hypothesized that in order to custom-make TS-1 product with different physico-chemical attributes, a unique binder or binder-combination thereof, with a superior silica yield and binding potential was vital. After several trials of varied combinations including with common binders used in the art had failed, Oligomeric silicates were tested. Oligomeric silicates are condensed, transparent liquids containing varying amount of SiO2 wt % (SiO2 content after hydrolysis). Of this group, Ethyl Silicate 40 [ETS-40 hereinafter] was chosen especially owing to its potential to yield higher silica (40 wt %) and being more economical than TEOS. Thereafter, adding a manipulative dimension to the recipe was explored with a view to provide for engineering the physico-chemical attributes of the shaped TS-1 product. During the course of testing several hypotheses to variegate one attribute of TS-1 with respect to others, a serendipitous find was encountered by these inventors wherein, it was concluded after several subsequent confirmatory tests that upon partial hydrolyses of ETS-40 the resultant ‘ethanol in combination with remaining ETS-40’ proved to be surprisingly and substantially superior to prior art binder precursors. It is pertinent to mention that ‘ETS-40-Silica-Ethanol-water-ammonia combination’ has never been used in the art as a binder precursor. This aspect is both novel and inventive as against the commonly accepted norm in prior art wherein ethanol generated by hydrolysis of TEOS had a counterproductive effect on the TS-1 forming process and therefore in all prior art processes the ethanol so generated had to be necessarily removed/drained out, before proceeding with the subsequent process steps of extrusion, tableting/pelleting etc., This prior art process step of removal of ethanol is not only time consuming but also has substantial cost implications. For making one Kg of TS-1 extrudates, 500 g of ETS-40 is used and up to 300 g of Ethanol is generated during hydrolysis in the present process, whereas in prior art process 715 g of Tetra ethyl ortho silicate (TEOS) is used and ˜540 g of Ethanol is generated. Due to multiple steps (such as spray drying, ethanol removal etc) involved in the prior art process, the processing costs are high compared to the present process. The cost of TEOS is approximately ˜1.5 times higher than ETS-40. Hence the present process is cost effective as both the raw material cost, processing cost are comparatively lower than prior art process. Removal of Ethanol is an additional step in the prior art processes, which is not only time consuming but also energy intensive.
Further interesting aspects about retention of Ethanol formed after the hydrolysis of ETS-40 during forming/shaping and its effect on the physico-chemical attributes of the shaped TS-1 were also studied. A number of experiments were carried out to regulate and re-regulate the hydrolysis stage-wise by administering varied amounts of ammonia into the reaction mixture/recipe and optionally secernating the said hydrolysis step. The hydrolysis conditions such as quantity of ammonia added and hydrolysis time significantly affect the resultant product properties,
It was observed that hydrolysis conditions influenced the properties in the shaped product. Yet another novel observation of this invention is that a slow progression of the hydrolysis reaction results in a superior product and also provides means to regulate the properties of the final shaped TS-1 product.
Thus, controlling the hydrolysis process and content of unique binder precursor of this invention provides for control over the final product properties such as Crushing strength (CS), Bulk density (BD) and pore volume distribution and Ti availability in the final product. In the case of ETS-40-Silica-Ethanol-ammonia-water binder recipe of this invention, the hydrolysis reaction is spilled over beyond the said shaping operation. By extending said hydrolysis process and by controlling the duration thereof it was found that the final physico-chemical parameters of the shaped TS-1 product can be engineered. This invention finds that the duration of hydrolysis, amount of ammonia added, amount of water added and drying time etc., influence the final product properties which have been illustrated in tables below a particular set of examples.
Thus, experiments with this novel recipe categorically indicated to provide for said engineering of physico-chemical attributes of shaped TS-1. Towards this objective, the present invention involves a drying step at elevated temperature such that by adjusting the drying time, said final physicochemical properties can be fine tuned and optimized to obtain the desired the crushing strength and other properties. This is novel and inventive over prior art processes.
These inventors went on to establish that the extrudate (and the granulated product) properties are influenced by the following, among other, factors:                (i) The amount of aq. NH3, the hydrolysing agent that is added;        (ii) The amount of water along with ammonia;        (iii) The amount of the unique binder precursor added        (iv) The duration of the operation of mixing of ETS-40 and dil. NH3, that is, the stirring period and        (v) The drying time;        
By regulating the above parameters, a surprising degree of control of the extrudate properties such as BD(Bulk Density), CS(Crushing strength) and Ti availability were observed by the invention, which is novel and inventive over prior art processes. The term ‘extrudate properties’ herein also includes the granulation, pelletized or tabletted product properties, unless otherwise required by the context. The shaping processes downstream of dough formation, namely, extrusion, tableting/pelleting and granulation are jointly referred to herein as fabrication processes or shaping process or forming process. Within the scope of the invention, the term fabrication includes all other feasible processes for handling/shaping of the dough material to obtain a discrete TS-1 catalyst product.
The binding action of ETS-40 is not adversely affected by presence of ethanol. As would be observed, ethanol is generated with the progress of the hydrolysis of ETS-40. Although prior art processes specifically eliminated ethanol in their respective processes, these inventors found that the presence of ethanol in the dough did not adversely affect the overall forming process. A higher ethanol concentration slows down the hydrolysis reaction and provides added scope for said manipulation/engineering of physico-chemical attributes of shaped TS-1. This is very advantageous because it does away with the necessity of removal of the generated ethanol, and/or other alcohols that is required when binders of the prior art are adopted. As mentioned, when the binder of this invention is adopted, the generated alcohol turns out to be a useful component of the TS-1 and binder mixture as the same creates the desired porosity in the product. Overall, removal of the alcohol by an energy-intensive operation is thus avoided.
References to the TS-1 and binder mixture herein are intended to include the forming compounds and/or other optional additives that may be added to form a part of the dough mixture.